1. Field of the Invention
The present invention is related to light emitting diode (LED) light extraction for opto-electronic applications. More precisely, the invention relates to the use of a structured emitting region suited for extraction of light usually trapped in the (Al,In,Ga)N thin film.
2. Description of the Related Art
(Note: This application references a number of different publications and patents as indicated throughout the specification by one or more reference numbers within brackets, e.g., [Ref. x]. A list of these different publications and patents ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications and patents is incorporated by reference herein.)
A number of publications and patents are devoted to the issue of light extraction from light-emitting semiconductor material. Light extraction can be achieved using geometrical optics effects, for example, using pyramids, outcoupling tapers or textured surfaces [8-14], or wave optics effects, for example, using microcavity resonances or photonic crystal extraction [15-18]. Special growth techniques such as pendeo or cantilever growths [19,20] or lateral epitaxial overgrowth [21,22] may also be used to achieve light extraction.
For more recent advances in the field of LED extraction, little is published. However, the concepts for light extraction using photonic crystal effects or Zinc Oxide (ZnO) pyramids are well-described in the applications listed in the Cross-Reference section above.
An efficient method for enhancing light extraction from an (Al,In,Ga)N LED consists in using photonic crystals to extract the light. Some proposals rely on the control of optical modes using a membrane system where most horizontal in-plane modes are suppressed by the photonic crystals, and the oblique modes do not radiate out of the membrane due to total internal reflection [23]. However, such structures suffer from two drawbacks: (1) the radiative emission rate is highly suppressed, requiring (2) emitting species with very high radiative efficiency, a property not easily achievable at room temperature.
Other implementations rely on using photonic crystals as a diffractive grating, positioned outside the emitting layer, as described in U.S. Provisional Application Ser. Nos. 60/866,014; 60/802,993; and 60/741,935; and U.S. Utility application Ser. Nos. 11/067,957; 11/067,910; and 11/067,956; which applications are listed in the Cross-Reference section above.
However, in such implementations, the interaction between the structure's guided modes and the crystal is rather weak, requiring a rather long photonic crystal, and consequently a large device, in order to extract most of the guided light. It is therefore highly desirable to incorporate the photonic crystal within the emitting layer, in order to achieve maximum interaction. However, this leads to three problems.
First, possible diminished radiative efficiency due to the large free surface of the active region which is degraded when etching the photonic crystal structure. Nonetheless, high quality material can still be obtained, by using annealing steps following the etching, as described in [24]. Alternatively, high quality material can also be obtained by direct columnar growth of the structure.
Another problem results from the lower photonic density of states in the etched region, which is a natural consequence of the diminished average index of refraction.
Finally, another source of very diminished emission might be due to the emitting layer having a lower index than the surrounding layer, and thus not supporting guided or localized modes.